" When working with a Faraday cage, remember that safety is of the utmost
importance. Any mains voltage electrical supply fed into the cage must
come
in via an isolating transformer so that the mains voltages are floating.
This will reduce the risk of electric shock if a chassis becomes live with
respect to the earthed cage. It should not be possible to reach any
'ordinary' mains powered sockets, switches or devices outside the cage
area
when you are within the cage. No external mains earth should be brought
into
the cage area. The Faraday cage should be provided with its own completely
separate electrical earth. Mains sockets within the cage area should not
contain an earth connection.


That is certainly not the way any screened room I have worked is has been
wired, and sounds extremely dangerous. Not having the earths connected
inside the room is potentially lethal should a case go live.This is a
particular problem with a lot of test equipment where the input filtering
causes the case to float to 1/2 mains voltage without an earth. An RCD will
not protect you in this situation because of the isolating transformer.

Imaging the situation where you are conducting emc tests with the UUT on an
earthed copper sheet but all the test equipment is floating at 120V!!

It is also a common requirement to have test equipment outside the screened
room but connected to equipment inside, isolating transformers and the lack
of mains earth would complicate this set up immensely.

All the installations that I have seen have not used isolating transformers,
just filtering on the incoming mains with a mains earth connection.
Protection being provided by an RCD on the mains input.

Jeff